Contamination of the surface of a metal, plastic, glass or other bare or finished part can arise from several sources. The surface may be exposed to soils from the environment in which it is located or used. Greases, oils and heavy-metal soaps are often applied to parts as lubricants to facilitate fabrication processes such as stamping, drawing, machining or annealing. Once a part is formed it may be stored for some period before it is used, or before its surface is treated. For example, metal surfaces usually require cleaning before phosphatizing, chromatizing or coating (such as painting or plating). Because unprotected metal surfaces may oxidize or tarnish over time, a stored metal part is sometimes coated with a protective oil to prevent surface oxidation or tarnish during storage.
Any such contaminants which remain on a surface prior to treatment of the surface can interfere with that treatment, for example, by preventing good adhesion of a coating to the surface. Accordingly, it is highly desirable that dirt, lubricants, metal oxides, rust, organic residues and protective oils be removed from surfaces before they are coated or painted.
While prior compositions for performing any one of these cleaning functions have been useful for their intended purposes, their use has been subject to some drawbacks. For example, oil, grease, rust or metal oxides can be cleaned from many metal surfaces by pickling. Pickling involves the application of a relatively concentrated solution of an acid (typically at least 15 percent concentration) such as hydrochloric acid, sulfuric acid, or phosphoric acid to the metal surface. Even though this will clean the metal oxide or rust from the metal surface, such solutions can corrode the metal surface itself. Moreover, due to the concentration of the acid solutions, the handling of pickling solutions requires a significant degree of care. Pickling solutions can corrode flesh, rubber hosing, pumps or concrete coming in contact with the solutions. There may also be significant environmental limitations on the disposal of pickling waste, due to its high acidity.
Abrading has also been employed to remove contaminants from the surface of a part. However, abrading typically entails an appreciable loss of material from the surface, and can result in unacceptable damage to the surface of the part, especially a part having a surface finish on it.
Different problems are encountered in other methods of removing oil and grease. Organic solvents have been successfully employed to remove grease and oil from metal and glass surfaces. However, the use of an organic solvent may pose a fire hazard, particularly if the solvent is deposited upon the metal surface from the vapor phase. Moreover, although the solvents used for cleaning are often recycled, they eventually must be disposed of, and the cost of disposal may not be inconsequential. Of course, organic solvents can attack many plastic surfaces and many surface coatings or finishes.
For purposes of cost and safety, it would be desirable to use aqueous solutions for surface cleaning. Concentrated solutions of strong alkalis have been used for degreasing, but their use has been subject to some drawbacks. Degreasing with strong alkalis may not be complete, while the use of strong alkalis (like the use of acid pickling solutions) can entail significant safety risks and environmental effects. Strong alkalis may also attack some glass and plastic surfaces, and some finishes. Accordingly, relatively neutral aqueous cleaners would be desirable.
Surface active agents (surfactants) such as detergents and wetting agents are known to be generally useful in dispersing grease and oil into aqueous suspension. Soaps (for example, the sodium or other light metal salts of fatty acids) and synthetic surfactants (cationic, nonionic and anionic surfactants) are commonly used for that purpose, either alone, or in combination with other materials. Surfactants are advantageous because they are usually less hazardous to use than other cleaning materials, and are often less costly than other materials to prepare, use or dispose of.
The use of surfactants in cleaning, in particular, the use of anionic surfactants, is subject to some drawbacks. Some solutions of these materials have a low enough pH or a sufficiently great ionic activity to corrode metal surfaces to which they are applied. The amount of dirt or other contaminants entrained in the oil or grease on a surface may affect the hardness of the cleaning solution applied to the surface, and thus affect the ability of the surfactant to clean the surface. Of course, the dispersant activity of a particular surfactant often varies with the type of grease or oil to be cleaned. Indeed, because many greases and oils are intended to be water-resistant, the use of an aqueous solution to clean many greases and oils is not common.
For example, Conoco Inc., Houston, Texas, sells a high temperature, corrosion-resistant grease under the name "HD Calcium Grease". HD Calcium Grease is a gelled hydrocarbon oil, containing calcium carbonate and calcium sulfate as gelling agents, as well as sodium nitride and zinc naphthenate as rust inhibitors, and an oxidation inhibitor. The product is formulated for use in salt water, fresh water or corrosive fluids, and has high water resistance and good rust inhibition. It is Applicants' understanding that it is the belief of Conoco that the HD Calcium Grease cannot be adequately cleaned from a metal surface with an aqueous cleaner.
Similarly, Witco Corp., Houston, Texas, sells a particular grease under the name "Prestige 741 AEP". The grease is made by Sun Chemical Corporation, Chester, South Carolina (product code #319451). The material is a reaction product of lithium 12-hydroxy stearate with Sun's 700 SSU base oil. The 700 SU base oil is primarily a mixture of more than 65 percent straight chain hydrocarbons, with the balance being naphthenic type hydrocarbons, with few aromatics and no significant additives. It is Applicants' understanding that aqueous cleaners are not recommended by Witco for removal of this grease from metal surfaces.
These greases are just two examples of the many greases and oils which are designed to resist dispersal in water. Of course, no surfactant is useful against all greases and oils, and certain greases and oils more tenaciously resist dispersal than others. While aqueous surfactants are intended to overcome the reluctance of hydrophobic greases to disperse, those surfactants most effective in dispersing the more tenacious oils and greases are unfortunately sometimes more likely to corrode the metal surface to which they are applied, precisely because they leave a cleaner surface than do less effective surfactants. The cleaner and thus less protected surface is more likely to be attacked by anything in the cleaning solution still in contact with the surface.
Corrosion inhibitors have been used in compositions having a purpose unrelated to cleaning grease and oil from metal surfaces, for example, in compositions useful for removing calcium and magnesium scale (insoluble deposits) or rust from metal equipment such as boilers or the like. One example is disclosed in U.S. Pat. No. 4,637,899 (Kennedy, Jr., Jan. 20, 1987), which employs an inhibitor comprising a sulfur-containing compound and at least one of an aliphatic pyridinium salt or an aliphatic quinolinium salt. A small but corrosion-inhibiting amount of this inhibitor is added to an aqueous solution of an organic acid or organic acid salt.
The property of acid corrosion inhibition has been attributed to 4-(2-hydroxyethyl)-2-piperazinone, in U.S. Pat. No. 4,814,443 (and its divisional U.S. Pat. No. 4,880,934), issued Mar. 21, 1989, to Dwayne S. Treybig and John M. Motes, assigned to the assignee of the present application. The specification of the patent is directed to processes for preparing this and similar materials from reacting hydroxyalkyl alkaline diamines with glyoxal. The specification notes the utility of the compound as a regenerative solvent for the desulfurization of flue gas, and as a binder between asphalt and fiberglass or a rock aggregate. There is no discussion, however, of the circumstances under which the material is operative to inhibit corrosion, nor of any compositions with which the material is compatible. Nor does the specification disclose or suggest how or whether the material might affect the utility of any composition to which it would be added.
It is therefore an object of the present invention to provide an improved, non-corrosive aqueous solution for cleaning grease and oil from a metal, plastic or glass surface.
It is another object of the present invention to avoid the need to use a concentrated acid, a concentrated alkali, or a hydrocarbon solvent to fully clean grease and oil from a metal, plastic or glass surface.
It is a further object of the present invention to provide a method to clean grease and oil from finished surfaces without damaging the finish on those surfaces.